Different healers have different healing styles.
Paladins tend to spam Flash of Light because they can. Paladins get relatively little benefit from FSR and they lack the high throughput heals other healers get, so they need to heal immediately. Flash of Light is also more (generally) more efficient than any heal a Druid can throw, so you’ve got it backwards here - you should be saving your mana and letting the Paladin top them off.
Shaman tend to spam Healing Wave (rank 1) when they’re not doing anything else because it stacks Healing Way (a healing buff on the target) and because it has a chance to proc an armor buff. They also have no choice about where Chain Heal bounces go, so they do a lot of ‘topping off’ even when they don’t mean to.
Priests are the ones you’re probably aiming at. In multi-healer situation, Priests should normally forget they even have fast heals and focus on drop huge heals infrequently, perhaps chased immediately with a Renew to optimize FSR. However, Priests also have a complaint to levy at Druids: stop healing people in the Priest’s party. Prayer of Healing is better than anything Druids can cast, so letting your Priest optimize their PoH usage is generally better than trying to use your own tools.
Lastly, we come to Druids.
Now, here’s the thing. People think of Druids as the “HoT healers”. Except, in Classic, they’re not. Renew is better than Rejuvenation and even Renew doesn’t move into the useful range until you’ve taken advantage of significant +healing scaling. We tend to assume that HoT have an efficiency advantage because theyr’e delayed healing, but this simply isn’t the case most of the time. It eventually is for Priests.
It’s never the case for Druids. Healing Touch is always more efficient and higher throughput than Rejuvenation. Regrowth has some value in specific builds, but it tends not to be the kind of heal you should be using in non-PvP situations. The main virtue of the HoT for Druid is in regards to Swiftmend.
So when you say “other healers, stop topping people off”, you’re making a faulty assumption. You’re assuming that you do it better than they do when, in fact, you almost certainly do it worse.